1. Field of the Invention
The present invention generally relates to oil and gas well (borehole) logging tools, and more particularly to an improved method of measuring the photoelectric absorption of geologic formations using neutron-induced, gamma-ray spectroscopy.
2. Description of the Related Art
Logging tools for measuring earth formation properties are well known, particularly those used in the location of underground petroleum products (oil and gas). Borehole logging instruments use various techniques to determine geophysical properties such as bulk density, porosity, water saturation, gas saturation, and lithology. The determination of the lithology of the formation, i.e., whether the predominant minerals are sandstone, limestone, dolomite, etc., is important in correlation and correction of the logging measurements and in describing reservoir parameters such as porosity typing and permeability.
Techniques for ascertaining formation properties include those involving the use of radiant (electromagnetic) energy. For example, gamma rays are commonly used to measure bulk density of a formation by detecting such radiation as it passes through the formation, and relating the amount of detected radiation to the electron density of the formation. See, e.g., U.S. Pat. No. 4,297,575. Gamma rays can be emitted continuously from a source in the borehole tool and propagate outward into the formation. This approach is known as gamma-gamma logging, because gamma rays originate in the tool, and the backscattered rays are thereafter detected in the tool. A typical gamma-ray source is cesium-137. Formation properties can be determined based on the count rate or intensity of the gamma rays that are received at detectors located in the tool. Usually at least two detectors (far and near) are used, which allows a measure of formation density that is essentially independent of the mudcake surrounding the tool (the mudcake is the layer of solid material lining the open borehole that has consolidated from the drilling fluid).
Instead of providing a radioactive gamma-ray source, gamma radiation can be produced in the formation in response to a high-energy neutron source (i.e., a neutron accelerator located in the borehole tool). This technique is referred to as induced gamma-ray logging. When the neutron source is pulsed, gamma rays are produced by one of three reactions:
inelastic scattering of fast neutrons, thermal neutron capture, and from the decay of radioisotopes created by neutron activation. The fast-neutron lifetimes are very small (a few microseconds) such that during the source pulse a mixed-energy neutron field exists. Shortly after the burst, all neutrons are thermalized (slowed down) and these thermal neutrons wander about until being captured, with a lifetime in the hundreds of microseconds. Gamma rays from inelastic scattering are produced in close proximity to the accelerator, and gamma rays from thermal capture are dispersed farther from the accelerator (up to tens of centimeters). See, e.g., U.S. Pat. No. 4,055,763.
Another common parameter which is measured in geophysical well log analyses is the formation photoelectric absorption cross-section. The photoelectric factor (proportional to the photoelectric absorption cross-section) is dependent on the average atomic number of the irradiated sample. The P.sub.e Factor measurements are used to create a profile of the photoelectric absorption cross-section in the formations traversed by the borehole. Quantitative methods have been devised in the prior art for measuring P.sub.e. These measurements are useful in determining the formation lithology because of their sensitivity to, e.g., calcium. There are many references in the prior art which provide methods to unambiguously transform derived constituents into lithology. See Fang et al., "Transformation of Geochemical Log Data to Mineralogy Using Genetic Algorithms," Log Analyst, vol. 37, no. 2 (1996).
One standard method for measuring P.sub.e is used in the borehole tool sold by Schlumberger Technology Corp. under the trademark LDT. The LDT tool is a gamma-gamma device, and its method of operation is further described in U.S. Pat. No. 4,048,495. The determination of the photoelectric factor is accomplished by measurement of the shape of the detected gamma-ray spectrum. With a properly calibrated LDT, P.sub.e can be inferred from the relationship between the count rates in a high energy window and a low energy window. A P.sub.e measurement can be further utilized to determine absolute elemental concentrations, as disclosed in U.S. Pat. No. 4,810,876. See also U.S. Pat. No. 4,628,202 which sets forth a variation on the LDT methodology, by developing an interrelationship between the photoelectric factor and density.
Conventional techniques for measuring P.sub.e suffer several disadvantages. First of all, they generally have a shallow depth of investigation into the formation; the irradiated sample is of a relatively small size. Smaller samples additionally cause the tool to be more sensitive to geometry factors, such as borehole rugosity and tool-pad contact, rendering the results less accurate. In cased wells, the very low-energy gamma rays used to measure the photoelectric factor with this prior art cannot penetrate the steel casing. Some of these problems might be mitigated using a neutron-induced spectroscopy system. Prior art systems such as the Schlumberger GST system or the Halliburton PSG system make measurements of individual formation chemical constituents or gamma yields via neutron-induced gamma spectroscopy. See Jacobson et al., "Elemental Spectral Gamma Log," Log Analyst, vol. 37, no. 1 (1996). These types of systems require many slow passes or stationary readings to acquire data with sufficient accuracy on the individual elements that make up the irradiated sample. Then through induction, the elemental gamma yields (and individual errors) and the elemental photoelectric cross-sections could then be combined to estimate the formation photoelectric factor. Similar to measuring miles with a yard stick, this method to determine the P.sub.e factor lacks efficiency and the level of complexity introduces complex systematic errors. Still other prior art systems such as the Halliburton TMD-L use neutron-induced gamma spectroscopy to generate qualitative ratios of Calcium-to-Silicon abundances, but the measurements are often ambiguous and cannot be directly applied to log analysis tools and paradigms that reference the formation P.sub.e factor. It would, therefore, be desirable to devise a method for measuring the formation photoelectric absorption in an earth formation, which overcomes the foregoing limitations.